Mental Math
Estimating and Using Models to Add and Subtract Decimals
Subtracting Decimals
100

Use compatible numbers or the associative property to add.

6.75 + 4.16 + 3.25 =

14.16

100

Round to the nearest whole to estimate.

19.7 - 6.9 =

3

100

55.25 + 2.98 + 16.30 =

74.30

100

82.7 - 5.59=

77.11

200

Use mental math to subtract. (Compensation would work well here.)

373.4 - 152.9 =

220.5

200

Round to the nearest tenth to estimate.

87.99 - 52.46=

35.5

200

39.48 + 26.7=

66.18

200

\$72.35 - \$6.19=

\$66.16

300

Solve:

201.96 + 38.7 + 0.84

241.5

300

2.4 - 1.7 = 0.7

300

Three bags of beads have masses of 10.3 grams, 5.23 grams, and 3.74 grams.  What is the total mass of all the beads?

19.27

300

Lila would like to take a ceramic class.  The class costs \$120.  She has saved \$80 so far. Write and solve an equation to find the amount that Lila still needs.

\$120 - \$80 = \$40; \$80 + d = \$120

400

James is buying school supplies.  He buys a notebook for \$2.45, a package of mechanical pencils for \$3.79, and an eraser for \$1.55.  Use mental math to fine how much he spent in all.

\$7.79

400

Martha bought an apple for \$0.89 and a drink for \$1.95.  Which is the best estimate of how much money she spent?

A.  \$2.00

B. \$3.00

C.  4.00

D. \$5.00

B

400

Choose all that are equal to 15.02.

A.  12.96 + 2.06

B.  0.56 + 14.64

C.  2.62 + 12.4

D.  1.22 + 1.8 + 12

E.  1. + 0.5 + 13.8

A and C

400

Miami receives 61.05 inches and Albany receives 46.92 inches of rainfall each year.  How much more rainfall does Miami receive than Albany?

14.13

500

In a week Karry ran 9.3 miles and Tricia ran 4.4 miles.  Use mental math to find how much farther Karry ran than Tricia. Explain how you determined your difference.

4.9

500

A restaurant bought 48.5 pounds of apples from a local orchard.  The next month, the restaurant bought another 65.3 pounds of apples and 24.5 pounds of pears.  How many pounds of fruit did the restaurant buy? Round to the nearest whole or use place Value blocks to help you solve the problem.

139; 138.3

500

Patrick has a 600 meter skein of yarn.  He used 248.9 meters to make a hat.  Does he have enough yarn left to make a scarf that uses 354.03 meters? Explain.

No, if you add the two amounts together their total is greater than 600 meters.

500

Which 2 subtraction problems have a difference of 10.2?

A.  12.05 - 2.03

B.  16.29 - 6.09

C. 36.1 - 25.9

D.  22.09 - 21.07

E.  10.82 - 9.8

B and C